© Blackwell Science Ltd, Geology Today, Vol. 17, No. 3, May–June 2001 93

CorrespondenceErratic errata

Your item ‘The future: erratic reasoning (possibly)’(Geology Today, v.17, p.12, 2001) invites one or twocorrections and comments.

The Preseli bluestone which sank in Milford Ha-ven will not confuse future research workers. It wasrecovered within a few days, using every moderncontrivance, and now rests on a Milford Havendockside!

As your report said, there has been some concernabout the £100 000 of Lottery Millennium Fundingwhich was devoted to this ill-conceived scheme. In-deed, in mid-May 2001, the MP for Preseli,Pembrokeshire, asked questions about the grant andfuture intentions for the stone. It was widely thoughtthat sledging a rather small version of a Stonehengebluestone over plastic matting, laid on modern as-phalt roads, hardly comprised a realistic reconstruc-tion! The sea journey was the crux of the problem.Had timber rafts been used, as your report suggested,it might just have been feasible, but the stone wasslung between two curragh-like craft, which provedtoo flimsy for the task.

The term ‘Little England beyond Wales’ (not just‘England’) refers to Pembrokeshire south of the Land-sker line, which separates the English from the Welshpart of the county. It does not include the whole pe-ninsula. The division was remarked upon in Eliza-bethan times by George Owen, and ‘Little England’became widely used after Edward Law’s published TheHistory of Little England beyond Wales in 1888.

Our local suggestion is that the stone should bebroken into several thousand fragments, polished andsold as souvenirs at Stonehenge! As it is impossiblelegally to obtain specimens of the Carn Meini dolerite,they could fetch a fair price.

J O H N E T H E R I N G T O NParc-y-Bont, Llanhowell, Pembrokeshire SA62 6XX

The Jurassic–Cretaceous boundary on theDorset coast

On the Dorset coast, the position of the Jurassic–Cretaceous boundary within the Purbeck sequence israther more complex than Jon D. Radley supposes

(Geology Today, v.16, p.216, 2000). At the moment,there is no internationally agreed type section inwhich to place the ‘golden spike’ to define this bound-ary in a fully marine, probably Tethyan, sequence ofsediments that were deposited in stable conditionswith little if any erosion and containing an abundantsuccession of well-preserved fossils, including ammo-nites. As this matter is far from resolved, Allen andWimbledon (Cretaceous Research, v.12, p.511, 1991)stated that in Wessex the Jurassic–Cretaceous bound-ary could either equate with the Pseudosubplanitesgrandis subzone at what Feist and others (Palaeontol-ogy, v.38, p.407, 1995) would consider to be towardsthe base of the Cypris Freestones, or ‘lie considerablyhigher, at or a little below the quasi-marine CinderBed’, which they suggested may even be diachronousand thus slightly transgress absolute time planes be-tween locations. There is insufficient evidence toshow if the Cinder Bed was generated by a true ma-rine transgression or by passive areal variations ofsalinity similar to those in the Baltic Sea (Allen, Pro-ceedings of the Geologists’ Association, v.109, p.197,1998). Owing to the marginal nature of suchsuccessions in north-west Europe, ammonites aremissing. Thus even with a combination of strati-graphic tools, correlation becomes problematic and isfurther complicated by the provinciality of Tethyanand Boreal marine biota (Hoedemaeker, GeologicaCarpathia, v.50, p.101, 1999) and the temporal gapsin most sequences.

However, when the British Geological Survey(BGS) came to revise the lithostratigraphy of the Pur-beck Limestone Group (Westhead and Mather, Pro-ceedings of the Geologists’ Association, v.107, p.117,1996), they found that the Cinder Bed, a very distinc-tive horizon mainly composed of oyster shells that isaround a metre or more thick, was easily mappableand in a reliable position for regional correlation asthe base of the Durlston Formation. This division ispart of a new five-member system for the Lulworthand Durlston Formations which replaces the informalthree-fold division of the Purbeck sequence, originallyconsidered to be wholly Jurassic. The 15 or so tradi-tional beds within the old system have been incorpo-rated into the five new members, which broadly re-flect sedimentary variations within the group, so thatthey can be used for lateral correlation and for inlandmapping of structurally complex areas with poor

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exposure, as these members form clear topographicalfeatures.

This study shows that, far from the Lulworth andDurlston Formations being an unnatural imposition,there were distinct lithological differences in the se-quences of southern Dorset above and below the Cin-der Bed, so that the Lulworth Formation varies moretowards the west, while above the Cinder Bed theDurlston Formation is more variable towards the east.This synthesis has been incorporated in the1 : 10 000 mapping of the area and the production ofthe completely revised 1 : 50 000 sheets forDorchester (328), Weymouth (341–2) and Swanage(342–3), which now include offshore geology. Onlyafter the 1 : 10 000 maps for the Isle of Purbeck werepublished in 1998 did the BGS adopt, in 2000, alower position for the Jurassic–Cretaceous boundaryon the new 1 : 50 000 maps. Thus it is only since Iwrote my articles about the geology of the Dorsetcoast at Lulworth and Worbarrow (Geology Today,v.14, p.71, 1998 & v.16, p.71, 2000), when for sim-plicity I wrote that most geologists take the Cinder Bedto mark the Jurassic–Cretaceous boundary, that theBGS has followed the present trend and placed thisboundary in the Cypris Freestones of the Mupe Mem-ber towards the base of the Lulworth Formation.

D A V I D N O W E L L2 Tudor Road, New Barnet,Herts. EN5 5PA

Holes at Lindal

Living in the Furness area, I was interested to readthe account of the 1892 rail subsidence at Lindal-in-Furness in Geology Today (v.16, p.79, 2000). When I

was a young boy travelling on the train, it was al-ways noticeable that the train slowed down to 10–15 mph whilst going through Lindal. We understoodthat it was because of the underlying mines, but didnot know of the 1892 incident and that a potentialvertical journey of 60–100 m was considered possi-ble. Talking to some of the residents of Lindal re-vealed a certain blasé approach to the possibility oftheir homes disappearing into geological history.Comments like ‘Ah well, another hole appeared inthe next field’ seemed to be passed off without a sec-ond thought.

As mentioned in the article, mining in the areawas for the iron ore hematite, which was restricted tothe Lower Carboniferous Limestone. The Furness orefields produced some of the richest iron ore in Europe,being 72–85% iron oxide. The discovery of the highlyproductive ore deposits was responsible for the in-credible growth of Barrow-in-Furness from a tinyfishing hamlet to a booming industrial town boasting12 Bessemer Converters, the largest steel works inEurope, within 50 years. The ore in the Lindal areawas found mainly on the hanging walls of the NW–SE-trending faults. It was the mining of one of theseveins that caused the 1892 subsidence. The accom-panying picture (Fig. 1) shows the 18.5-m-wide en-trance to Daylight Hole, the site of one of these orebodies. The main chamber is 83 m long, 41.5 m wideand in places over 18.5 m high, depending upon thelevel of the lake.

D E R E K L E V I S T O N2 Long Row, Marshside,Kirkby-in-Furness,Cumbria LA17 71UP

Going for the hard stuff

Some reflections on the quarry items, ‘Quarry re-jected’ and ‘Where now for aggregates?’ (Geology To-day, v.17, pp.6–7, 2001): Long ago I tried to assessthe relative values of hard limestone, granite, basaltand gritstone to the aggregate industry and to societyin general, and concluded (New Scientist, 13 May1989, pp. 56-60; Mendip Limestone Quarrying, a Con-flict of Interests, Somerset Books, 1994, pp.27-40) thathard limestone was marginally the most valuablerock type to the industry but much the most valuableto society.

In Britain, Carboniferous Limestone is the domi-nant hard limestone. It is special when left in situ forits capacity to form spectacular scenery in a non-mountainous context (Cheddar Gorge in the Mendips,Malham Cove and Gordale Scar in the Pennines), forits value to the tourism industry (the Yorkshire Dales

Fig. 1. Daylight Hole,Lindal. Note the man in thewhite shirt for scale.

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